The present invention relates to the design and manufacture of a gauge for checking conformance of surface features of a manufactured component.
A conformance gauge is used as an inspection tool to check that a manufactured component conforms to its nominal design within permitted limits. In particular the invention concerns the manner in which the gauge itself is manufactured in order to minimise manufacturing variations which could compromise the inspection procedure. In a particular embodiment described to illustrate the invention, a conformance gauge is used to check the positions of cooling holes formed in the leading edge of a cast nozzle guide vane for a gas turbine engine.
In a gas turbine engine, an annular array of nozzle guide vanes is located in the exit annulus of the combustion chamber to impart xe2x80x98swirlxe2x80x99 to the hot gas stream entering the first turbine stage. To protect these vanes from the hot gas each has a number of cooling holes formed along or close to its leading edge through which relatively cool air from the engine compressor is supplied to form a protective film over the exterior surfaces (gas-washed surfaces) of the vane. The cool air is fed through a passage in the interior of the vane and vented through holes formed through the walls of the vane. The cool air then forms a protective film of air across the gas-washed surfaces that prevent impingement of the hot gases and consequent damage to the vane.
It is important to minimise the amount of cooling air required as it has a direct impact on the efficiency of the gas turbine engine. To this end, cooling holes are positioned only in critical areas such as the leading edge of the vane. Furthermore, the cooling hole exit apertures may be shaped to maximise the efficiency of the protective film. As a result, the shape and position of each cooling hole, particularly the location and shape of the exit, is critical and it is essential therefore that the cooling hole exit apertures are within prescribed tolerance limits calculated at the design stage.
Determining that these cooling holes are positioned within the prescribed tolerance limits is difficult. At the design stage, each hole is dimensioned from a datum located at the centre of the vane. Once the vane has been manufactured however, this datum is inaccessible and so the holes must be checked relative to at least one other external feature. Typically the gas washed surfaces of the vane are used, because they share the same datum as the cooling holes and, being a high tolerance feature, minimise additional error. Nevertheless all surfaces and edges are toleranced and none, neither surface nor hole, can be relied upon to be exactly at its nominal design position, hence the need to check during inspection.
The current method for checking the hole locations relative to the gas washed surfaces is to use a xe2x80x9csighting gaugexe2x80x9d made of clear material, such as a polycarbonate sheet vacuum formed over a buck; which may be either an example of a cast vane or a purpose built die. Lines are drawn or scribed onto the vacuum formed sheet to indicate the tolerance limits of the cooling hole positions. During inspection, a manufactured vane is inserted into the gauge and the positions of the cooling hole exit apertures in the vane are checked against the scribed limit lines on the gauge. If the cooling holes do not lie within the scribed tolerance limits, the component is rejected.
A good fit between a gauge and manufactured component is important to successful inspection. While the vacuum forming process ensures that the internal contact surface of the gauge is the converse of the surface of the back on which the gauge was formed, manufacturing variations in the buck mean that the contact surface of the gauge almost certainly is not an accurate nominal surface and is inevitably biased toward one or the other of the tolerance limits. As a result, the surfaces of the gauge and vane do not interlock accurately in every pairing and the small variations in the gas-washed surface of the vane and the positions of the formed cooling holes can result in acceptable blades being rejected, or vice versa.
The current gauge is made in three steps, each of which inevitably introduces errors. When the buck is manufactured, errors are introduced, (particularly where the buck is an actual vane, subject to the manufacturing tolerances of a production component). Further errors are introduced when the polycarbonate is vacuum formed about the buck due to shrinkage and distortion. Still further errors are introduced when marking the polycarbonate due to the difficulty of assessing nominal positions of the holes from the design, represented by drawings which can show only a two-dimensional view of the vane.
While each error may be small in isolation, the effect is cumulative. Deviation in the form of the gauge serves to reduce the accuracy of interlock between vane and gauge reducing the accuracy of the gauge. This inaccuracy is compounded by the errors in marking. Further inaccuracies are then introduced when the features are inspected as a result of parallax error. In use, it is important that the inspector views a given feature at the correct angle through the gauge. This gives scope for error or misinterpretation, a problem compounded by optical distortion arising from viewing through the polycarbonate.
Also the use of a cast buck to manufacture the gauge, whether an actual vane or special blank, involves an inevitable time delay at a time when manufacturing processes are making increasing use of simultaneous engineering techniques. The time delay involved when design changes are introduced may be no longer acceptable.
It is an objective of the present invention to provide gauges with improved accuracy, reduced cost and lead times.
According to the present invention a method of design and manufacture of a conformance gauge for checking conformance of at least one selected feature of a manufactured component within tolerance limits of the nominal dimensions of the feature, the component having been designed using CAD tools, through use of which there has been created a first CAD data file containing the co-ordinates of the nominal dimensions of the component design and the said at least one selected feature, the method of designing the conformance gauge comprising creating a second CAD data file containing the co-ordinates of the nominal dimensions of the conformance gauge including an inspection feature corresponding to the at least one selected feature of the component design, wherein the co-ordinates of the inspection feature are derived from or are copied from the co-ordinates of the at least one selected feature contained in the first CAD data file.
The term xe2x80x9cconformal gaugexe2x80x9d is used in this specification to mean a gauge which relies at least in part upon one or more surfaces engaging with corresponding component surfaces to be inspected and upon this fitting of one with the other to ensure that the gauge is correctly aligned with the component to be inspected.
In a modem component design process using computer aided design equipment (CAD) a CAD file for the gauge is created directly from a CAD model of the manufactured component, preferably using the data contained in the original file. This allows the gauge to share the same three-dimensional geometry as the inspected component and ensure a conformal fit between gauge and inspected component. A rapid prototyping apparatus is then used to create a gauge directly from the CAD file.
In effect, the gauge is produced from a nominal example of the component to be inspected, thereby eliminating inaccuracies introduced by the additional step in which a manufactured component is used as a mould onto which the gauge is vacuum formed. As previously mentioned, these inaccuracies arise from both manufacturing tolerances and from the vacuum forming process itself. Another advantage is that the gauge design and manufacture process does not need to be delayed until an example of the inspected component is manufactured, thus facilitating simultaneous engineering of component and gauge. Where the construction of new gauges is necessary, such as damage or expiry of gauges in use, lead time is reduced to less than a day. This reduces considerably the requirement for spare gauges kept in stock. Cost is also substantially less than the original method.
In a preferred embodiment of the invention, a number of inspection apertures or slots are created in the CAD file of the gauge. Each such aperture or slot is shaped to bound a region in which a feature which conforms to the prescribed tolerance must lie. The gauge is then created using the rapid prototyping process and apparatus.
Because the inspection apertures so created are integral with the manufacture of the gauge, the need to mark the gauge after manufacture and the attendant inaccuracy is avoided. Also tougher materials (such as nylon or metal in the case of laser sintering) may be used as the gauge no longer needs to be transparent or vacuum formed.
As well as improvements in gauge quality, the ease of use is improved because the region within which a feature must lie is bound by gauge material flush with the surface of the component. This eliminates parallax error during inspection and, because the inspection means is now viewing via an aperture in the gauge, no optical distortion comes into play.